In gas turbines, the two-armed gas collection or bifurcated pipe between the combustion chamber housing and the inlet flange of the turbine blades is subject to an extreme stress and increased wear due to temperature, pressure and corrosion during hot operation.
The combustion air is compressed in a compressor to a high pressure, and an essential portion is used for combustion in the two combustion chambers, and a smaller portion is used to cool the hot metal parts.
The essential percentage of the O.sub.2 of the air is used for oxidation in the combustion chambers by burning a carbon carrier. Nitrogen remains in the exhaust gas as a ballast and is additionally brought to high temperatures under high pressure and it flows from the combustion chambers into the bifurcated pipe and from there into the turbine to the turbine inlet blades and sets same into increased rotation.
The gas collection or bifurcated pipe consists of an iron-nickel-base material. This is attacked by high pressure and especially by an elevated gas temperature, with oxygen oxidizing the metal surface.
The alloying elements of the Ni-base alloy, such as aluminum, chromium or the like, reduce a further oxidation by forming solid oxide coatings.
However, this passive oxide coating does not prevent nitrogen from penetrating, so that the nitrogen can form nitrides or carbonitrides with the above-mentioned alloying elements over time, and the formation of these nitrides and carbonitrides is thermodynamically facilitated by the higher pressure of the gas.
The consequence is that depending on the alloying constituents and the solubility of N.sub.2, AlN (nitrides) and/or Cr carbonitrides may be formed under the oxide coating.
This leads to the binding of the aluminum concentration in the metal, on the one hand, so that the oxidation resistance decreases and AlN needles and/or Cr carbonitrides are formed, which leads to an embrittlement of the metal.
This mechanism takes place not only in the combustion space of the bifurcated pipe, but also in the outer surface, which come into contact with the cooling air and which cannot always be cooled to the extent that the said gas-metal reaction can take place.
As a high-temperature corrosion protection, the entire inside of the gas collection pipe is lined with an MCrAlY monolayer, which is characterized by increased chromium and Al content. A nickel-based spray powder containing 31% of Cr, 11% of Al and 0.6% of Y is used here.
The high-temperature corrosion and oxidation coating develops a high resistance potential against oxidation and the nitrogen content increase and consequently an increased high-temperature corrosion and oxidation resistance because of the increased Cr and Al contents in conjunction with yttrium.
Heat-insulating coatings (TBC=Thermal Barrier Coating) are applied as an additional corrosion and heat protection on the surface of the inner cone of the gas collection pipe, to which the hot gas is admitted.
The heat-insulating coating is a plasma-sprayed coating system consisting of a bond coat and a ceramic top coat, which brings about the heat insulation of the coating system.
The bond coat is used, besides for bonding the top coat, also to avoid the high-temperature corrosion and oxidation of the material. To optimally assume both functions, this bond coat consists of a two-layer MCrAlY coat, a so-called bond coat A and B.
Bond coat A is a ductile MCrAlY coating with reduced chromium and aluminum content in order to guarantee long-term optimal bonding to the substrate.
Bond coat B is an MCrAlY coat with increased chromium and aluminum content. As a result, the increase in the nitrogen content in the base material is prevented, besides the increased high-temperature corrosion and oxidation resistance.
The top coat consists of a ZrO.sub.2 --Y.sub.2 --O.sub.3 ceramic and brings about the heat insulation of this coat because of its lower thermal conductivity.
High-temperature-and corrosion-resistant protective coatings made of alloys and containing essentially nickel, chromium, cobalt, aluminum and an admixture of rare earth metals for gas turbine components, which require high corrosion resistance at medium and high temperatures and are in direct contact with the hot exhaust gases from the combustion chamber, have been developed and introduced on the market in many different compositions.
Multiple protective coatings for metal objects, especially gas turbine blades, have been known from WO 89/07159. Based on the discovery that there are two different corrosion mechanisms which are of significance for the life of such objects, two protective coatings arranged one on top of another are proposed, of which the inner coating offers protection against corrosive effects at temperatures of 600.degree. C. to 800.degree. C. and the outer coating is optimized for attacks at temperatures of 800.degree. C. to 900.degree. C. In addition, a thermal barrier coating may also be present as an outermost coating. A diffusion coating with a chromium content greater than 50% and with an iron and/or manganese content exceeding 10% is preferred as the first coating, and an MCrAlY coating, which contains, e.g., about 30% of chromium, about 7% of aluminum and about 0.7% of yttrium and is applied by plasma spraying under reduced pressure, is preferred as the second coating.
A protective coating, especially for gas turbine components, which possesses good corrosion properties in the temperature range of 600.degree. C. to about 1,150.degree. C., has been known from WO 91/02108. The protective coating contains (in weight percent) 25-40% of nickel, 28-32% of chromium, 7-9% of aluminum, 1-2% of silicon, 0.3-1% of yttrium, the rest being cobalt, at least 5%; and unavoidable impurities. Various optional components may be added. The properties of the protective coating can be further improved by adding rhenium. This effect appears even upon the addition of small quantities. A range of 4-10% of rhenium is preferred. P The coatings may be applied by plasma spraying or vapor deposition (PVD) and are especially suitable for gas turbine blades made of a superalloy based on nickel or cobalt. Other gas turbine components, especially in the case of gas turbines with a high inlet temperature exceeding, e.g., 1,200.degree. C., may also be provided with such protective coatings.
A nickel or cobalt metal alloy, to which a protective coating against increased temperature attacks and corrosive attacks of hot gases from the combustion chamber of a gas turbine is applied, has been known from WO 96/34128.
The three-layer protective coating comprises a first bond coat consisting of an MCrAlY composition against the base metal to be protected and a second anchoring coating against the outer oxide coating.
A metal substrate based on a nickel or cobalt alloy, to which a protective system against increased temperature, corrosion and erosion is applied, has been known from WO 96/34129.
The protective system comprises an intermediate coating, consisting of a bonding coating against the Ni substrate and an anchoring coating against the outer ceramic coating based on zirconium oxide. The outer ceramic coating acts as a thermal barrier coating.
A device, especially a gas turbine means, with a coating of components of the device, has been known from DE 42 42 099.
Components in gas turbine systems and similar devices, which come into contact with hot gases during their operation, are provided with a coating there, which has both a corrosion protective action and a catalytic action. Components in the temperature range higher than 600.degree. C. are provided with a coating that has an oxidation-catalyzing action, and components in a temperature range of 350.degree. C. to 600.degree. C. are provided with a coating having a reduction-catalyzing action. Mixed oxides with perovskite or spinel structure based on LaMn are used for the coating of the first type, and mixed oxides of the same structure based on LaCu are used for the coating of the second type.